Wafer level packaging of MEMS

ABSTRACT

A MEMS device is formed by applying a lower polymer film to top surfaces of a common substrate containing a plurality of MEMS devices, and patterning the lower polymer film to form a headspace wall surrounding components of each MEMS device. Subsequently an upper polymer dry film is applied to top surfaces of the headspace walls and patterned to form headspace caps which isolate the components of each MEMS device. Subsequently, the MEMS devices are singulated to provide separate MEMS devices.

FIELD OF THE INVENTION

This invention relates to the field of microelectronic mechanicalsystems (MEMS) devices. More particularly, this invention relates topackaging in MEMS devices.

BACKGROUND OF THE INVENTION

MEMS devices may require a cavity, also referred to as a headspace, in apackage to allow proper operation. Forming a package for the MEMS devicewith a headspace that is hermetically sealed from the outsideenvironment at a desired cost may be problematic.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to amore detailed description that is presented later.

A MEMS device may be formed by applying a lower polymer film to topsurfaces of a plurality of MEMS devices on a common substrate, andpatterning the lower polymer film to form headspace wall surroundingcomponents of each MEMS device. Subsequently an upper polymer dry filmis applied to top surfaces of the headspace walls and patterned to formheadspace caps which isolate the components of each MEMS device.Subsequently, the MEMS devices are singulated to provide separate MEMSdevices.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A through FIG. 1G are perspective views of an exemplary MEMSdevice, depicted in successive stages of fabrication.

FIG. 2A through FIG. 2E are perspective views of another exemplary MEMSdevice, depicted in successive stages of fabrication.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention is described with reference to the attachedfigures. The figures are not drawn to scale and they are provided merelyto illustrate the invention. Several aspects of the invention aredescribed below with reference to example applications for illustration.It should be understood that numerous specific details, relationships,and methods are set forth to provide an understanding of the invention.One skilled in the relevant art, however, will readily recognize thatthe invention can be practiced without one or more of the specificdetails or with other methods. In other instances, well-known structuresor operations are not shown in detail to avoid obscuring the invention.The present invention is not limited by the illustrated ordering of actsor events, as some acts may occur in different orders and/orconcurrently with other acts or events. Furthermore, not all illustratedacts or events are required to implement a methodology in accordancewith the present invention.

A MEMS device may be formed by applying a lower polymer layer to topsurfaces of a plurality of MEMS devices on a common substrate, andpatterning the lower polymer film to form headspace walls on each MEMSdevice which surround components of each corresponding MEMS device toprovide a headspace for each MEMS device. The lower polymer layer may bea dry film material or a spin coat material. Pillars and/or fins insideof the headspace and/or outside of the headspace may be concurrentlyformed with the headspace walls to support subsequently formed headspacecaps. Internal dividers which separate each headspace into two or morecompartments may also be formed concurrently with the headspace walls.

Subsequently an upper polymer dry film is applied to the headspace wallsand patterned to form the headspace caps on each MEMS device. Thecombination of the headspace wall and headspace cap on each MEMS devicesisolates the components of that MEMS device. Scribe lines betweenadjacent MEMS devices on the common substrate are free of the headspacewalls and headspace caps. The MEMS devices may have bondpads on the topsurface; the headspace walls are formed so that the bondpads are outsidethe headspace, and the bondpads are free of the headspace walls andheadspace caps. Alternatively, the MEMS devices may havethrough-substrate-vias (TSVs) with bondpads on a bottom surface of thesubstrate; in this case the headspace walls and headspace caps mayextend to edges of the MEMS device.

Subsequently, the MEMS devices are singulated to provide separate MEMSdevices. After singulation, the MEMS devices may be encapsulated in apackaging material such as mold compound.

FIG. 1A through FIG. 1G are perspective views of an exemplary MEMSdevice, depicted in successive stages of fabrication. Referring to FIG.1A, the MEMS device 100 is formed concurrently with at least one otherMEMS device 102 on a common substrate 104. The substrate 104 may be, forexample, a silicon wafer, a ceramic carrier, a glass or sapphire sheet,or other substrate appropriate for forming the MEMS device 100. At leastone scribe line 106 separates the MEMS device 100 from the at least oneother MEMS device 102.

The MEMS devices 100 and 102 include components 108 disposed at a topsurface 110 in a central region of each MEMS device 100 and 102, andbondpads 112 disposed at the top surface 110 proximate to edges of theMEMS devices 100 and 102. The components 108 may include, for example,cantilevers with proof masses, as depicted in FIG. 1A. Other components108 are within the scope of the instant example. In the instant example,sacrificial layers supporting the components 108, such as sacrificialpolymer layers, silicon dioxide layers or polysilicon layers, areremoved before proceeding to the next fabrication step.

Referring to FIG. 1B, a lower polymer layer 114 is applied to the topsurface 110 of the MEMS devices 100 and 102. In one version of theinstant example, the lower polymer layer 114 may be a dry film which isapplied as a sheet. In another version, the lower polymer layer 114 maybe a spin coat material which is applied as a mixture of polymer andsolvent by dispensing the mixture onto the top surface 110 and spinningthe substrate 104 to distribute the mixture and subsequently baking themixture to remove at least a portion of the solvent. More than onedispense and spin cycle may be used to obtain a desired thickness of thelower polymer layer 114. The lower polymer layer 114 may include, forexample, photoresist, photoactive polyimide or photoimageable epoxy. Thelower polymer layer 114 may be, for example, 5 microns to 100 micronsthick, depending on the components 108 of FIG. 1A and requirements ofthe MEMS device 100.

The lower polymer layer 114 is exposed with ultraviolet light to defineareas 116 for subsequently formed headspace walls. Instances of thelower polymer film which use a negative tone photolithographic process,such as polyisoprene photoresist, polyimide, or SU-8 epoxy, are exposedin the areas 116 for the headspace walls. Instances of the lower polymerfilm which use a positive tone photolithographic process, such asnovolak resin photoresist, are exposed outside the areas 116 for theheadspace walls. The areas 116 may include areas for pillars, fins ordividers.

Referring to FIG. 1C, a develop process removes unwanted polymer fromthe lower polymer layer 114 of FIG. 1B to form headspace walls 118 onthe top surfaces 110 of the MEMS devices 100 and 102. The headspacewalls 118 surround the components 108 in each of the MEMS devices 100and 102. The headspace walls 118 are not formed in the scribe lines 106.Internal pillars 120, external pillars 122, internal fins 124 and/orexternal fins 126 may be formed concurrently with the headspace walls118. Internal pillars 120 are located inside the headspace walls 118 andare separate from the headspace walls 118. External pillars 122 arelocated outside the headspace walls 118 and are separate from theheadspace walls 118. Internal fins 124 are located inside the headspacewalls 118 and are contiguous with the headspace walls 118. External fins126 are located outside the headspace walls 118 and are contiguous withthe headspace walls 118. The headspace walls 118 may be baked to releasevolatile material which was present in the lower polymer layer 114 orintroduced by the develop process.

Referring to FIG. 1D, an upper polymer dry film 128 is applied to theheadspace walls 118. The upper polymer dry film 128 may include, forexample, SU-8 epoxy, 10 microns to 300 microns thick. Presence of theinternal pillars 120, external pillars 122, internal fins 124 and/orexternal fins 126 may advantageously reduce sagging or distortion of theupper polymer dry film 128 upon application to the headspace walls 118.

Referring to FIG. 1E, the upper polymer dry film 128 of FIG. 1D ispatterned by exposure to ultraviolet light and subsequently developed toform headspace caps 130 on the headspace walls 118. On each MEMS device100 and 102, the headspace wall 118 and the headspace cap 130 combine toisolate a headspace 132 over the components 108. The headspace caps 130do not extend into the scribe lines 106 or extend over the bondpads 112.

Referring to FIG. 1F, the MEMS devices 100 and 102 are subsequentlysingulated, for example by sawing or scribing, so as to produce separateMEMS devices 100 and 102. The headspace walls 118 and the headspace caps130 may advantageously protect the components 108 of FIG. 1E during thesingulation process, for example from saw kerf debris or scribeparticles.

Referring to FIG. 1G, the MEMS device 100 may optionally beencapsulated, for example by mold compound in a leaded package, asdepicted in FIG. 1G. A metal film such as aluminum may be formed overthe headspace walls 118 and the headspace caps 130 before encapsulationwith the mold compound to further protect the MEMS device fromcontaminants. The headspace wall 118 and the headspace cap 130 mayadvantageously protect the components 108 of FIG. 1E during theencapsulation process from the mold compound. Other schemes of packagingthe MEMS device are within the scope of the instant example.

FIG. 2A through FIG. 2E are perspective views of another exemplary MEMSdevice, depicted in successive stages of fabrication. Referring to FIG.2A, the MEMS device 200 is formed concurrently with at least one otherMEMS device 202 on a common substrate 204. At least one scribe line 206separates the MEMS device 200 from the at least one other MEMS device202.

The MEMS devices 200 and 202 include components 208 disposed at a topsurface 210 in a central region of each MEMS device 200 and 202. Thecomponents 208 may be, for example, beam resonators 208 as depicted inFIG. 2A. In the instant example, sacrificial material 234 such asphotoresist, polyimide, silicon dioxide or polysilicon, is present ineach MEMS device 200 and 202, for example under beams of the beamresonators 208. In the instant example, through-substrate-vias (TSVs)236 are formed through the substrate 204 to electrically connect thecomponents 208 to bondpads on a bottom surface of the substrate 204.

Referring to FIG. 2B, a lower polymer layer is applied to the topsurface 210 of the MEMS devices 200 and 202 and patterned to formheadspace walls 218. The headspace walls 218 surround the components 208in each MEMS device 200 and 202. In the instant example, dividers 238are formed concurrently with the headspace walls 218 so as to formseparate compartments in the headspace 232 of each MEMS device 200 and202. The headspace walls 218 do not extend into the scribe lines 206,but may overlap the TSVs 236 of FIG. 2A.

Referring to FIG. 2C, the sacrificial material 234 of FIG. 2A is removedafter the headspace walls 218 are formed, so as to release thecomponents 208. The sacrificial material 234 may be removed, forexample, using a downstream asher. The sacrificial material 234 isremoved without breaching the headspace walls 218. Initial dimensions ofthe headspace walls 218 may be selected to compensate for material lossduring removal of the sacrificial material 234.

Referring to FIG. 2D, an upper polymer dry film is applied to theheadspace walls 218 and patterned to form headspace caps 230. On eachMEMS device 200 and 202, the headspace wall 218 and the headspace cap230 combine to isolate a headspace 232 over the components 208. Theheadspace caps 230 do not extend into the scribe lines 206.

FIG. 2E shows a bottom surface 240 of the substrate 204 of the MEMSdevices 200 and 202. The TSVs 236 of FIG. 2A terminate in bondpads 212on the bottom surface 240. The MEMS devices 200 and 202 are subsequentlysingulated, and may optionally be encapsulated.

What is claimed is:
 1. A method of forming a first MEMS device,comprising the steps of: forming a scribe line in a substrate, whereinthe scribe line separates an area for said first MEMS device from anarea for a second MEMS device; forming a first component of the firstMEMS device on a top surface of the substrate in the area for the firstMEMS device; forming a second component for the second MEMS device onthe top surface of the substrate in the area for the second MEMS device;applying a lower polymer layer to the top surface of the substrate;patterning the lower polymer layer to form a first headspace wall on thesubstrate in the area for the first MEMS device surrounding the firstcomponent and to form a second headspace wall on the substrate in thearea for the second MEMS device surrounding the second component;applying an upper polymer dry film to the first headspace wall and thesecond headspace wall; patterning the upper polymer dry film to form afirst headspace cap on the first headspace wall in the area for thefirst MEMS device and to form a second headspace cap on the secondheadspace wall in the area for the second MEMS device, so that the firstheadspace cap and the first headspace wall isolate the first componentof the first MEMS device and the second headspace cap and the secondheadspace wall isolate the second component of the second MEMS device;and singulating the first MEMS device from the second MEMS, wherein thestep of patterning the lower polymer layer includes forming a firstpillar inside the first headspace wall and separate from the firstheadspace wall and a second pillar inside the second headspace wall andseparate from the second headspace wall.
 2. The method of claim 1,wherein said step of patterning the lower polymer layer comprises thesteps of exposing the lower polymer layer in areas for the firstheadspace wall and the second headspace wall with ultraviolet light, andsubsequently performing a develop process to remove unexposed polymermaterial from the lower polymer layer.
 3. The method of claim 2, whereinthe lower polymer layer comprises polyimide.
 4. The method of claim 2,wherein the lower polymer layer comprises epoxy.
 5. The method of claim1, wherein the step of patterning the upper polymer dry film comprisesthe steps of exposing the upper polymer dry film in areas for the firstheadspace cap and the second headspace cap with ultraviolet light, andsubsequently performing a develop process to remove unexposed polymermaterial from the upper polymer dry film.
 6. The method of claim 5,wherein the lower polymer layer comprises epoxy.
 7. The method of claim1, further comprising the step of encapsulating the first MEMS device inmold compound.
 8. The method of claim 1, further comprising the step ofremoving a sacrificial layer contacting the first component of the firstMEMS device and the second component of the second MEMS device, afterthe step of patterning the lower polymer layer and prior to the step ofapplying the upper polymer dry film.
 9. The method of claim 1, whereinthe scribe line is free of polymer material from the lower polymer layerafter the step of patterning the lower polymer layer is completed. 10.The method of claim 1, wherein the scribe line is free of polymermaterial from the upper polymer dry film after the step of patterningthe upper polymer dry film is completed.
 11. The method of claim 1,further comprising forming bondpads on the top surface of the substrate,wherein the bondpads are free of polymer material from the lower polymerlayer after the step of patterning said lower polymer layer iscompleted.
 12. A method of forming a first MEMS device, comprising thesteps of: forming a scribe line in a substrate, wherein the scribe lineseparates an area for said first MEMS device from an area for a secondMEMS device; forming a first component of the first MEMS device on a topsurface of the substrate in the area for the first MEMS device; forminga second component for the second MEMS device on the top surface of thesubstrate in the area for the second MEMS device; applying a lowerpolymer layer to the top surface of the substrate; patterning the lowerpolymer layer to form a first headspace wall on the substrate in thearea for the first MEMS device surrounding the first component and toform a second headspace wall on the substrate in the area for the secondMEMS device surrounding the second component; applying an upper polymerdry film to the first headspace wall and the second headspace wall;patterning the upper polymer dry film to form a first headspace cap onthe first headspace wall in the area for the first MEMS device and toform a second headspace cap on the second headspace wall in the area forthe second MEMS device, so that the first headspace cap and the firstheadspace wall isolate the first component of the first MEMS device andthe second headspace cap and the second headspace wall isolate thesecond component of the second MEMS device; and singulating the firstMEMS device from the second MEMS device, wherein the step of patterningthe lower polymer layer includes forming a first fin inside the firstheadspace wall and contiguous with the first headspace wall and a secondfin inside the second headspace wall and contiguous with the secondheadspace wall.
 13. The method of claim 12, wherein said step ofpatterning the lower polymer layer comprises the steps of exposing thelower polymer layer in areas for the first headspace wall and the secondheadspace wall with ultraviolet light, and subsequently performing adevelop process to remove unexposed polymer material from the lowerpolymer layer.
 14. The method of claim 13, wherein the lower polymerlayer comprises polyimide.
 15. The method of claim 13, wherein the lowerpolymer layer comprises epoxy.